1. Field of the Invention
The present invention relates to a multilayer ceramic substrate, a method for manufacturing the multilayer ceramic substrate, and a method for reducing the warping of the multilayer ceramic substrate and, in particular, to a method for manufacturing a multilayer ceramic substrate using a so-called non-shrink process, a multilayer ceramic substrate manufactured by this manufacturing method, and a method for reducing the warping of the multilayer ceramic substrate.
2. Description of the Related Art
In the manufacture of a multilayer ceramic substrate by a so-called non-shrink process, a composite laminate is first prepared. The composite laminate includes a green ceramic laminate and first and second shrinkage control layers disposed on opposite surfaces (first and second main surfaces, respectively) of the green ceramic laminate. The green ceramic laminate includes a plurality of ceramic green layers each containing a material that can be fired at a low temperature (hereinafter referred to as a low-temperature firing ceramic material). The first and second shrinkage control layers contain a powder formed of an inorganic material that cannot be sintered under sintering conditions of the low-temperature firing ceramic material.
The composite laminate is then fired under the sintering conditions of the low-temperature firing ceramic material. In this firing process, since the inorganic material is not substantially sintered, the first and second shrinkage control layers will not substantially shrink. The first and second shrinkage control layers can therefore reduce the shrinkage of the ceramic laminate along the main surfaces, thus increasing the dimensional accuracy of the ceramic laminate and consequently increasing the dimensional accuracy of a multilayer ceramic substrate including the ceramic laminate. In the firing process, it is known that the low-temperature firing ceramic material chemically reacts with the inorganic material along an interface between a shrinkage control layer and a ceramic green layer to form a reaction layer.
The first and second shrinkage control layers are then removed, for example, by abrasive blasting, to manufacture a desired multilayer ceramic substrate.
According to a method for manufacturing a multilayer ceramic substrate by a non-shrink process, therefore, a multilayer ceramic substrate can be manufactured with high dimensional accuracy particularly along the main surfaces. However, variations in the distribution of conductive portions, such as conductor films and via conductors, in a green ceramic laminate, which is to become a multilayer ceramic substrate, and in the thickness and composition of a ceramic green layer may cause the warping of the multilayer ceramic substrate in the firing process. In particular, a surface conductive film on a main surface of a green ceramic laminate is known to have a large effect on the warping of a multilayer ceramic substrate.
Japanese Unexamined Patent Application Publication No. 2001-60767 has proposed altering the thickness of a first shrinkage control layer relative to a second shrinkage control layer to reduce the warping. PCT International Publication No. WO 2002/043455 has proposed altering the particle size of an inorganic material powder in a first shrinkage control layer relative to that in a second shrinkage control layer.
However, even when the warping in the firing process is reduced as described above, removal of shrinkage control layers may cause new or additional warping. This will be described below with reference to FIG. 6.
FIG. 6 is a cross-sectional view of a composite laminate 1 after a firing process is completed and a first shrinkage control layer 6 (broken line) is removed.
The composite laminate 1 includes a ceramic laminate 2, which is composed of a plurality of ceramic layers 3 each formed of a sintered low-temperature firing ceramic material. Conductor films and via conductors associated with the ceramic laminate 2 are not shown in FIG. 6. A first shrinkage control layer 6 and a second shrinkage control layer 7 are disposed on opposite surfaces (a first main surface 4 and a second main surface 5, respectively) of the ceramic laminate 2. A first reaction layer 8 and a second reaction layer 9 are disposed between the ceramic laminate 2 and the first and second shrinkage control layers 6 and 7.
First, removal of the first shrinkage control layer 6 by abrasive blasting in the direction indicated by an arrow 10 releases the compressive stress acting on the ceramic laminate 2, thus causing a convex warp on the first main surface 4. Second, removal of the second shrinkage control layer 7 also releases the compressive stress acting on the second main surface 5, thus making the ceramic laminate 2 substantially flat.
However, in such removal of the shrinkage control layers 6 and 7, variations in the distribution of conductive portions, such as conductor films and via conductors, in the ceramic laminate 2 and the thickness and composition of the ceramic layers 3 may cause non-uniform compressive stress, thus leaving a warp of a multilayer ceramic substrate composed of the ceramic laminate 2. Furthermore, it is also known that such warping is greatly affected by surface conductive films disposed on the main surfaces 4 and 5 of the ceramic laminate 2.